Recent developments in solid free-from fabrication (SFF) techniques have now made it possible to manufacture complex shapes with advanced material systems. It is now feasible to generate parts with specific gradient material and hard metal or ceramic materials. However, much of the attention in SFF has been directed towards attaining volumetric precision. An outstanding issue that must be addressed is that of surface improvement. Currently, SFF techniques generate surfaces with roughness comparable to rough machining operations. With the Laser Engineered Net Shaping (LENS) and the Selective Laser Sintering (SLS) processes, which are among several Rapid Prototyping (RP) methods capable of producing dies and molds, the surface roughness is at the high end of the process scale, i.e. Ra is approximately equal to 25-50 μm (comparable with rough machining and casting). Consequently, post-processing is required to bring the surface quality to specification. The cost of post-processing by finish machining or polishing hard materials (which are typically materials used for tool and die construction) easily exceeds the cost of rough machining. In some cases, the cost of finishing Titanium and hard ceramics can be 60% of the total processing cost. The time and labor involved in finish machining and polishing can also be high compared with the initial machining time, adding to the turnaround delays. Thus, if the potential of SFF technology is to be realized for rapid production of tools and dies and for re-manufacturing valuable components, methods for improving the quality of surfaces produced by these processes must be developed.
The most common use of SFF currently is in the development of prototypes. The potential of these methods in producing tooling is evident, and is a topic of intense research. A variety of SFF technologies have been investigated and developed, including Sterolithography (SLA), Selective laser sintering (SLS), Laser engineered net shaping (LENS), Laminated object manufacturing (LOM) and 3D printing. While these methods have been useful in niche applications, none of these are currently capable of achieving net shapes when the tolerance and surface requirements are stringent. In most cases, while these methods can achieve volume deposition with reasonable precision, the surface finish of parts leaves much to be desired and constrains the direct use of the products so produced. Subsequent processing to improve the surface finish is usually required and this can be as complex and costly as the finish machining of equivalent parts produced by subtractive methods. Further, although these methods have made it possible to design parts with intricate features, these may be difficult to post-process by conventional surface improvement methods such as grinding, lapping, or honing due to constrained tool access. In these cases, it may be useful to use the original CAD files and the same setup as the original fabrication environment to improve the surface quality. In the case of repair and remanufacture of costly components, if the surface roughness specifications can be achieved during the metal deposition operations, expensive and time consuming re-fixturing and finish machining operations can be eliminated, with the savings ultimately passed on to consumers.
Prior art methods of laser smoothing include laser ablation and laser polishing. Laser ablation is a process whereby ultra-short pulsed lasers are focused onto target surfaces, causing impurities or defects to be vaporized or ablated from the work piece. Laser ablation results in negligible heat-affected zones, and hence it has been widely used in thin-film deposition micro-machining and surface cleaning. The main difference between laser ablation and the micro-melting technique proposed here is that the former seeks to vaporize material selectively whereas the latter seeks to re-distribute material selectively. This difference results in substantial implementation differences, the mechanism for controlling the two processes are quire different. In ablation, most of the incident energy is carried away by the ejected material. In micro-melting the energy input has to be precisely directed and controlled so as to guide the material flow from local “hills’ to “valleys’. In ablation, kerf formation must be minimized. This is clearly not as significant an issue in micro-melting. Ablation seeks to process small material volumes selectively whereas micro-melting also seeks to processes small volumes, but since surface improvement is not usually a local or spot requirement, processing is extensive.
Laser polishing is perhaps most closely related to processes proposed here. Laser polishing and micro-melting both offer similar benefits, i.e. applicability to a wider variety of materials, little to no damage to the base metal and precise control. Several studies on the use and success of laser polishing with different material systems, including glass, polymers, ceramics, diamonds, and metals have been reported in the literature. Laser polishing has found widespread use mostly in the semiconductor industry and the proposal for modeling and investigation the underlying process in the context of general surface improvement is new.